In wavelength modulation spectroscopy (WMS) the wavelength and the intensity of the light of a tunable light source, usually a continuously tunable laser such as a diode laser, is modulated with a frequency f0, while the wavelength is swept over an interaction feature of a sample to be measured, for example a molecular absorption line in a gas sample. As the light interacts with the sample, for example propagates through a gas sample, wavelength dependent interaction such as absorption converts some of the wavelength modulation into an intensity modulation of the light. Thus, the light will have an overtone spectrum generated by the interaction process, the harmonic content of the spectrum being dependent on the interaction feature, for example the width and shape of the molecular absorption line in the gas and the etalons in the system. When the light then impinges onto a measuring detector, for example a photodiode, the measuring detector output contains AC components at the modulation frequency f0 and its higher harmonics Nf0 (N=2, 3, 4, etc.). Demodulating the measuring detector output at one of said higher harmonics Nf0 shifts the measurement from frequencies near DC, where the light source is noisy, into a higher frequency range, where the noise is lower, thus improving the measurement sensitivity.
In difficult measurement conditions, however, the light may be further modulated by a time dependence of the optical transmission of the system. An example are in-situ measurements of trace gas constituents in combustion environments, where varying dust loads, high temperature, gas turbulence etc. modulate the light in the kHz range.